Novel slip agents and polypropylene films prepared therefrom

ABSTRACT

Novel polypropylene films are prepared from a composition comprising polypropylene and an N,N—N,N′-bis-alkylene fatty amide having the general structural formula R—CO—NH—(CH 2 ) n —NH—CO—R 1  wherein R and R 1  are the same or different alkyl or alkenyl groups having about 15 to 21 carbon atoms, n is an integer from 2 to 4 and —CO— is a carbonyl group. Such films are found to have improved utility for gravure printing applications.

[0001] This invention relates to a novel polypropylene compositionhaving specific utility in the manufacture of oriented polypropylenefilms and to polypropylene films prepared therewith. More specifically,it relates to polypropylene compositions containing certain fatty acidamides not heretofore known to be useful as components in polypropylenecompositions.

BACKGROUND INFORMATION

[0002] In the manufacture of polypropylene films, it is common practiceto include, in the polymer formulation, additives referred to as slipagents. These additives migrate to the surface of the film and decreasethe coefficient of friction between the film and the metal rollers overwhich it is passed during processing, thus facilitating the processingof the film. They also decrease the coefficient of friction betweenlayers of the film when it is wound into rolls thereby facilitatingunwinding of the rolls for further processing.

[0003] The slip additives employed in most commercial polypropylenefilms are relatively high molecular weight fatty acid amides. The mostwidely used fatty acid amides are erucamide, an unsaturated 22 carbonamide (13-docosenamide) and behenamide (docosanamide), the saturatedanalogue of erucamide. Both of these compounds are readily available,naturally occurring materials. They are normally provided as mixturescontaining a small amount of other amides containing about 18 to 20carbons.

[0004] Erucamide works quite well as a slip agent, but it is not favoredby some producers and converters as it is relatively volatile and extracare is required during film manufacture and conversion to avoid havingquantities of the additive escape from the film and plate out onprocessing equipment, thereby causing a clean-up problem. For thisreason, some producers prefer to use behenamide as the slip agent. Asslip agents, erucamide and behenamide appear to perform substantiallyequally well but behenamide is less volatile and therefore filmscontaining behenamide are easier to handle during processing since thebehenamide does not escape and plate out on the processing equipment.

[0005] Despite the inconveniences encountered with erucamide-containingfilm, erucamide is the fatty acid amide slip agent of choice for thosewho wish to print the film using the well known rotogravure printingtechnique. In the rotogravure process, ink is applied to the printsurface from a gravure cylinder containing the desired image asdepressions on its surface. Ink is applied to the cylinder and thesurface of the cylinder is wiped by a doctor blade to remove excess ink.The film to be printed is then contacted by the gravure cylinder and theimage is thus transferred from the cylinder to the film. While the filmis in contact with the cylinder, a small amount of erucamide isdeposited on the raised portion of the cylinder. Since erucamide issoluble in the alcohol or ketone ink solvent, it is dissolved by thesolvent in the next application of ink and is removed by the next swipeof the doctor blade. Thus, no build-up of erucamide occurs on the doctorblade or on the gravure cylinder.

[0006] Behenamide, on the other hand, lacking the carbon-carbonunsaturation found in erucamide, is substantially less soluble in thealcohols and ketones found in printing inks. Thus, when, in the courseof rotogravure printing, some of the behenamide is deposited on theprinting cylinder, it is not dissolved in the printing ink and wiped offwhen the next application of ink and wiping by the doctor blade takesplace. As a result, the behenamide is merely wiped off by the next swipeof the doctor blade and substantially all of it remains on the blade.After multiple swipes of the blade, a behenamide build-up forms on thedoctor blade and on the printing cylinder, eventually reaching a pointat which the blade is prevented from wiping the surface of the printingcylinder clean. This build-up causes streaks to form on the printedfilm, requiring an interruption of the job in order to clean the doctorblade.

[0007] It will be immediately apparent that a slip additive that doesnot cause the film processing problems associated with erucamide butwhich allows the film to be rotogravure printed without causing theproblems associated with behenamide would be a welcome contribution tothe polypropylene film art. In accordance with this invention, there hasbeen discovered a slip additive that accomplishes this objective.

[0008] It is an objective of this invention to provide orientedpolypropylene films that exhibit improved properties in rotogravureprinting as compared to films presently known to the art. it is likewisean objective of this invention to provide film-forming compositions forpreparing such films.

BRIEF DESCRIPTION OF THE INVENTION

[0009] In accordance with this invention, it has been found that certainbis-fatty acid amides exhibit a combination of properties that makesthem highly satisfactory as slip agents in oriented polypropylene films.In particular, this combination of properties make them highlysatisfactory as slip agents in films that are to be printed inrotogravure printing operations.

[0010] Briefly stated, the invention is a composition comprised of apolymer or copolymer of propylene and an N,N′-bis-alkylene fatty acidamide having the general structural formula R—CO—NH—(CH₂)_(n)—NH—CO—R¹wherein R and R¹ are the same or different alkyl or alkenyl groupshaving about 15 to 21 carbon atoms —CO— is a carbonyl group and n is aninteger from 2 to 4 said N,N′-bis alkylene fatty acid amide beingpresent in an amount from about 0.05 to about 0.5% by weight based onthe combined weight of said amide and the polypropylene.

[0011] In another aspect, the invention contemplates an oriented filmcomprised of a polymer or copolymer of propylene and anN,N′-bis-alkylene fatty amide having the general structural formulaR—CO—NH—(CH₂)_(n)—NH—CO—R¹ wherein R and R¹ are the same or differentalkyl or alkenyl groups having about 15 to 21 carbon atoms, n is aninteger from 2 to 4 and —CO— is a carbonyl group said N,N′-bis alkylenefatty acid amide being present in an amount from about 0.05 to about0.5% by weight based on the combined weight of said amide and thepolypropylene.

[0012] A preferred embodiment of the invention is a composite filmcomprised of a polypropylene core having, on at least one of itssurfaces, a film layer comprised of a composition comprisingpolypropylene and up to about 0.5% of an N,N′-bis-alkylene fatty amidehaving the general structural formula R—CO—NH—(CH₂)_(n)—NH—CO—R¹ whereinR and R¹ are the same or different alkyl or alkenyl groups having about15 to 21 carbon atoms, n is an integer from 2 to 4 and —CO— is acarbonyl group, said N,N′-bis alkylene fatty acid amide being present inan amount from about 0.05 to about 0.5% by weight based on the combinedweight of said amide and the polypropylene.

DETAILED DESCRIPTION OF THE INVENTION

[0013] As used in the description of this invention and in the attachedclaims, the term “film” refers to a stand-alone film, i.e. a film of asufficient thickness to have the strength and other characteristicsrequired to have utility in packaging and other applications withouthaving to be united with another film to support it. Stand-alonemonolayer films are seldom seen in commerce. However, the compositionsof the invention are suitable for use in such films.

[0014] Oriented polypropylene films of commerce are usually composite,i.e. multilayer, structures in which a core layer of a thicknesssufficient to impart stand-alone properties to the overall structurecarries one or more thin functional layers on its surface(s). The term“film” will also be used to refer to these thin functional layer filmsalthough they do not have sufficient thickness to stand alone.

[0015] Composite polypropylene films are typically comprised of ahomopolypropylene core having one or more functional layers on itssurfaces. Functional layers can act, e.g., as heat or cold seal layers,as receiving layers for printing or other decorative material or asbarrier coating receiving layers. In these cases, it is not requiredthat the polymer in the surface layer be the same as that employed inthe core, although it may be the same polymer formulated differently toserve the functional role that it is to serve in the completed film.Frequently, the surface layer will be of a different polymer that hasbeen found to perform the desired function more satisfactorily thanwould the polymer employed in the core.

[0016] When reference is made herein to polypropylene, it is intended toindicate a crystalline (isotactic) propylene homopolymer or a copolymerof propylene with ethylene or an α-olefin having 2 to 5 carbon atoms inan amount insufficient to have a significant effect on the crystallinityof the polypropylene. Typically, the comonomer will be ethylene in anamount of 6% or less.

[0017] Suitable polypropylenes are the commercially available isotacticpolypropylenes having a melt flow rate between about 2 and about 10dg/10 min at 230° C. and 2.16 Kg. load and a DSC melting point of about160 to 166° C. One polypropylene that can be used is the isotactichomopolymer having a melt flow rate of about 3.5 dg/10 min at 230° C.and 2.16 Kg. load, available from Aristech Chemical Corporation,Pittsburgh, Pa.. Suitable polypropylenes are also available fromMontell, Inc. Wilmington, Del., Exxon Chemical Company, Baytown, Tex.,and Fina Oil and Chemical Co., Deer Park, Tex.

[0018] Since different layers are formulated differently, it is possiblethat different slip agents may be used in the different layers or thatsome layers contain no slip agent. Thus, it is possible and desirable,in cases where problems are encountered when using conventional slipagents, e.g. in the rotogravure printing process as described above, tolimit the use of the bis-amides of the invention to the functional layerwhere its presence yields the desired advantage. In such cases, where inthe past the conventional slip agents have been added to the core layerand allowed or forced to migrate into the functional layers, it is foundthat the presence of the bis amides almost completely prevents themigration of the conventional slip agent, e.g. erucamide or behenamidefrom the core to the functional layer where it can cause a problem.

[0019] The slip additives employed in the compositions and orientedfilms according to this invention are N,N′-bis- fatty acid amides havingthe general structural formula R—CONH—(CH₂)_(n)—NH—CO—R¹ wherein R andR¹ are the same or different alkyl or alkenyl groups having about 15 to21 carbon atoms, n an integer from 2 to 4 and —CO— is a carbonyl group.The compounds employed in the compositions of this invention arecharacterized by having a relatively low degree of volatility and arelatively high degree of solubility in alcohols, ketones and esters.

[0020] Compounds meeting the above definition and description include,by way of example, N, N′ bis-ethylene stearamide, N,N′-bis ethyleneoleamide, N,N′-bis ethylene behenamide, N,N′-bis ethylene erucamide,N,N′-bis propylene stearamide, N,N′-bis butylene oleamide.

[0021] The N,N′-bis-alkylene fatty acid amide is present in thecompositions of the invention in a concentration of about 0.05 to about0.5% by weight based on the total weight of the composition. Preferably,the concentration of the slip additive is between about 0.05 and about0.25% and most preferably between about 0.5 and about 0.2%.

[0022] The slip agents of the prior art such as erucamide or behenamideare sufficiently volatile that a measurable portion of them alwaysmigrates from the interior of the film and forms a layer of slip agenton the film's surface, whether the film is a composite or a monolayer.The amount present on the surface can be measured by a techniquereferred to as Attenuated Total Reflectance Infra-Red Spectroscopy(ATRIR). When the ATRIR spectrum of a film sample stored at roomtemperature is compared to that of a film stored for 16 hours at 55° C.,an increase in the amount of a prior art slip agent is detected. (Suchheat aging is commonly applied to simulate film aging conditions.) Theamount of the slip agents of this invention that migrates to the surfaceof a film cannot be measured by ATRIR. Their presence on the filmsurface is demonstrated only by their efficacy as slip agents.

[0023] The presence of conventional slip agents on the surface of a filmcan also be detected by rinsing the film surface with a solvent andtesting for the slip agent by High Pressure Liquid Chromatography.Similarly, no accumulation of the slip agent of the invention canmeasured on the film surface by this technique, whereas it is readilydetected in the solvent employed to wash the surface of a prior artfilm.

[0024] In prior art practice, slip agent is frequently incorporated intothe core layer of composite films, which are then heat treated to forceit to migrate to the surface layers. In other cases, if the slip agentis present only in a surface layer, it tends to migrate into the corelayer although, as a rule, it is not needed there. In either case, thismigratory tendency results in slip agent being present in locationswhere it is not needed. Moreover, it also requires the use of greaterquantities of slip agent than are needed in order to assure thatsufficient quantities are present in the area where the real needexists.

[0025] It has been found that the slip agents of this invention exhibita very low tendency to migrate between layers or even within a layer ofthe film. Thus, a slip agent incorporated in a surface layer does notmigrate into the core layer, nor does it migrate and accumulate on thesurface of a layer as do the slip agents of the prior art. When testedby the ATRIR test or the surface rinse procedure mentioned above noaccumulation of the slip agent is detected on the surfaces. The lowmigratory character of these slip agents causes them to remain in thelayer into which they are incorporated.

[0026] Despite the absence of measurable amounts of the slip agent onthe film surface, the slip agents of the invention are substantiallyequal to those of the prior art in decreasing the coefficient offriction (COF) of the film surface. Thus, the COF measured on thesurface of a prior art film containing erucamide as the slip agent isabout 0.15 to 0.35. That measured on a prior art film containingbehenamide is about 0.15 to 0.35. The COF measured on the surface of afilm of this invention is generally between about 0.15 and about 0.45.

[0027] The low migratory character of the slip agents of the inventionalso leads to other favorable effects on film processing. For example,the problem of vaporization encountered with erucamide and to a lesserextent with behenamide is eliminated. Also, as will be apparent from theabove, in the preparation of composite films, it is not always necessaryto have a slip agent in the core layer. Since the slip agent of theinvention will not migrate into the core layer, one can put only theamount actually needed to give the desired slip properties in thesurface layer and it will not migrate into the core layer and reduce thelevel in the surface layer below that required to give the surface ofthe film the proper slip quality.

[0028] As indicated hereinabove, the compositions of the invention haveparticular utility in the manufacture of rotogravure printable orientedpolypropylene film thanks to the unique combination of good alcohol,ketone and ester solubility and low volatility exhibited by theN,N′-bis-alkylene fatty acid amides. Due to their low degree ofvolatility, these compounds do not vaporize from the surface of the filmduring manufacturing and conversion. Due to their relatively high degreeof solubility in the solvents that are normally employed as solvents forprinting inks, whatever small amounts these compounds that might be onthe film surface does not form a build-up on the doctor blade duringrotogravure printing. Films prepared using these compositions thuscombine the favorable properties of conventional films containingbehenamide during the manufacture and conversion of the film with thefavorable properties of those containing erucamide for use inrotogravure printing.

[0029] In yet another aspect, the slip agents employed in the films ofthe invention are found to be improved candidates for use as cold sealfilms. Cold seal films are prepared by treating the films with a coldsealable adhesive such as a rubber cement and many applications forprinted films are applications in which cold seals are preferred overmore conventional heat seals. In prior art usage, particularly whenerucamide is employed as the slip agent, it is found that the high levelof the slip agent on the film surface weakens or even, in extreme cases,destroys cold sealability. Using the slip agents of this invention atthe levels specified, very little or no harmful effect on cold seal isseen.

[0030] In addition to the slip agents of this invention, conventionaladditives, in conventional amounts, can also be included in the films.Suitable conventional additives include, by way of example,antioxidants, pigments, orientation stress modifiers, flame retardants,antistatic agents, antiblocking agents, and antifoggants. Any suchadditives that do not interfere with the functioning of the slip agentcan be present.

[0031] In any embodiment of the films of the invention, the film can beeither clear and transparent or it can be opaque. Typically,polypropylene films are rendered opaque by loading the core layer with avoid forming opacifier such as calcium carbonate. Such opacifiers causeopacity by forming microvoids in the polymer matrix during the drawingoperation. Other types of opacifiers which cause opacity simply byincreasing the optical density without voiding can also be employed.Typical of such an opacifier is titanium dioxide.

[0032] The total film thickness, for either the single or multiple layerfilms, is preferably in the range of about 0.25 to about 1.5 mil, i.e.about 25 to about 150 gauge. In the multilayer embodiment, the corelayer preferably has a thickness of about 23 to about 40 gauge, whilethe other layers preferably each have a thickness of about 2 to about 10gauge.

[0033] To prepare a printable surface, the surface of the film that isto be printed is subjected to an oxidation treatment to impart a degreeof polarity to the surface, which ensures good adhesion of the printedinformation to the film. Suitable oxidation treatments include, by wayof example, corona discharge, flame and acid etching. The preferredtreatment is with corona discharge or with flame. The use of flame andcorona, seriatim, is also known and frequently employed.

[0034] The oxidation treatment is preferably carried out to a degreesufficient to create a surface tension on the film surface equal toabout 35 to about 60 dynes/cm. A preferred surface tension is about 35to about 40 dynes/cm.

[0035] As stated hereinabove, composite films can have, in addition tothe printable surface, other functional layers on the surface oppositethe print layer. If the layer on the side opposite the print layer is abarrier coating receiving layer, that layer will also be subjected to anoxidation treatment to approximately the same degree as is applied tothe print layer.

[0036] While the invention has been described with emphasis on itsutility in rotogravure printing, it will be apparent that other types offilms, including functional layers, can also benefit from the lowvolatility and the lower degree of migratory tendency of the slip agentsof the invention. One type of such film is metallizable film where thegreater migratory tendency and volatility of conventional slip agentstends to result in a greater amount of slip agent on the film surface,which can sometimes interfere with bonding of the metal coating to thefilm surface. The same effect is also noted with cold seal coatingsapplied to the film surface.

[0037] Polypropylene films according to the invention can be preparedusing film forming and orientation methods and techniques well known tothe plastic film art. Thus the films can be prepared using the tubularor bubble process wherein a tube of film is extruded through a circulardie and is drawn and inflated simultaneously to effect biaxialorientation. Likewise, they can be prepared by the tenter process inwhich a flat sheet of the film is drawn from the die at a multiple ofthe lineal rate of extrusion to effect longitudinal orientation and isthereafter drawn laterally by a preselected multiple of its originalwidth to effect lateral orientation. As a rule, films prepared by thebubble process are drawn uniformly 6× by 6× or 7× by 7×. When the tenterprocess is employed, the draw is usually carried out to 5× in themachine direction and 10× in the cross direction. These draw ratios arealso suitable for the films of the invention, although other ratios canbe employed if desired.

[0038] In the examples that follow, a series of biaxially drawn threelayer composite films were prepared on a 2.5 meter tenter. The filmswere cast from a three layer die at 265° C. onto a chill roll maintainedat 40° C. The cast films were drawn 4.8× in the machine direction at120° C. and then 10.4× in the cross direction at 120° C. Finally, theywere heat set at 160° C. to a relaxation of about 8%. All samples werethen corona treated on a covered roll corona treater at 35° C. and 20amps.

EXAMPLE 1

[0039] A three layer composite film comprised of a homopolymer core of0.69 mil thickness containing 0.07% by weight behenamide, a 0.02 milhomopolymer layer on one surface containing 0.25% crosslinked methylsesquioxide particles and, on the other surface, a 0.04 mil layer of acopolymer of polypropylene and 4.5% ethylene (Fina 8573 from Fina Oiland Chemical Co., Deer Park, Tex.). This latter layer, intended to serveas a print layer, contained 0.10% by weight of N,N′-ethylenebisoleamide.

EXAMPLE 2

[0040] Example 1 was repeated except that the concentration of N,N′ethylenebis oleamide was 0.05% by weight.

EXAMPLE 3

[0041] Example 1 was repeated except that the concentration of N,N′ethylenebis oleamide was 0.20%. 11 color Cerutti press (North AmericanCerutti Corporation, Pittsburgh, Pa.) Examples 1 to 6 were printed withManders inks (Manders Premium, Inc., Niles, Ill.) thinned to a #22 Zahncup with a 1:1 ethyl alcohol/n-propyl acetate solvent. Examples 7 and 8were printed with Sun NULAM and Sun SUNPLI inks thinned with Sun SL02and SL05 solvent systems. (Here again we should have more informativedescriptions).

[0042] All samples were evaluated on commercial quality print jobs andevaluated against commercially accepted controls as standards. In bothevaluations, Examples 5 and 8, which contained no N,N′-ethylenebisoleamide, exhibited unacceptable streaking and/or ghosting. The films ofExamples 1,2,3,4,6 and 7 which contained the N,N′-ethylenebis oleamide,exhibited no streaking or ghosting.

[0043] Two aspects of the cold seal properties of the film samples wereexamined. In order for the film to be able to be unwound from a rollwhere the cold seal layer is in contact with the other surface of thefilm on which no such adhesive is present, it must release readily fromthat layer. Then, when a cold seal is formed by bringing two areas ofthe cold seal layer together, it must not separate without applicationof a significant pulling force. These two aspects are referred to asrelease strength, which must be low, and the cold seal strength, whichmust be high.

[0044] To test these two parameters, a typical commercially availableadhesive, Findlay C1099 (Findlay Adhesives, Wawatosa, Wis.) was appliedto the print layer of a specimen of the film using a #7 Meyer rod. Thecoated film was then dried at 93° C. for 30 seconds. The dried film wasthen covered with a release film and cut into six individual samples.These were placed in a blocking jig at 100 psi and blocked at 100 psifor 16 hours at 50° C.

[0045] To test cold seal release strength, the release film was pulledfrom the specimen in an Instron tensile tester and the force to requiredto remove the film was measured.

[0046] To test cold seal adhesion strength, a specimen was cut in twoalong its longitudinal axis and the two halves were brought together,coating to coating, and pressed together at 80 psi for ½ second. Withinone minute following sealing, the two pieces of film were pulled aparton the Instron tensile tester. Triplicate measurements were made foreach specimen.

[0047] The cold seal release strength and the cold seal adhesion areboth found to be comparable to that of conventional commercial filmsprepared with behenamide and erucamide slip agents.

What is claimed is:
 1. A composition comprised of polypropylene and anN,N′-bis-alkylene fatty acid amide having the general structural formulaR—CO—NH—(CH₂)_(n)—NH—CO—R¹ wherein R and R¹ are the same or differentalkyl or alkenyl groups having about 15 to 21 carbon atoms —CO— is acarbonyl group and n is an integer from 2 to 4, said N,N′-bis alkylenefatty acid amide being present in an amount from about 0.05 to about0.5% by weight based on the combined weight of said amide and thepolypropylene.
 2. The composition according to claim 1 wherein thepolypropylene is a copolymer of ethylene and propylene containing up toabout 6% by weight of ethylene.
 3. The composition according to claim 2wherein the N,N′-bis-alkylene fatty acid amide is N,N′-bis-ethyleneoleamide.
 4. A polypropylene film comprised of a composition comprisingpolypropylene and an N,N′-bis-alkylene fatty acid amide, having thegeneral structural formula R—CO—NH(CH₂)_(n)—NH—CO—R¹ wherein R and R¹are the same or different alkyl or alkenyl groups having about 15 to 21carbon atoms —CO—is a carbonyl group and n is an integer from 2 to 4,said N,N′-bis alkylene fatty acid amide being present in an amount fromabout 0.05 to 0.25% by weight based on the combined weight of said amideand the polypropylene.
 5. A film according to claim 4 wherein thepolypropylene component is a copolymer of ethylene and propylenecontaining up to about 6% by weight of ethylene.
 6. A film according toclaim 5 wherein the N,N′-bis-alkylene fatty acid amide isN,N′-bis-ethylene oleamide.
 7. A composite film comprised of a corelayer of polypropylene having one or more functional layers on itssurfaces, at least one functional layer being a layer comprisingpolypropylene and an N,N′-bis-alkylene fatty acid amide having thegeneral structural formula R—CO—NH—(CH₂)_(n)—NH—CO—R¹ wherein R and R¹are the same or different alkyl or alkenyl groups having about 15 to 21carbon atoms —CO— is a carbonyl group and n is an integer from 2 to 4,said N,N′-bis alkylene fatty acid amide being present in an amount fromabout 0.05 to 0.25% by weight based on the combined weight of said amideand the polypropylene:
 8. A composite film according to claim 7 whereinthe polypropylene component of the said at least one functional layer isa copolymer of ethylene and propylene containing up to about 6% byweight of ethylene.
 9. A composite film according to claim 8 wherein theN,N′-bis-alkylene fatty acid amide is N,N′-bis-ethylene oleamide.